Ballasted railroad tracks, developed nearly two centuries ago, are built using a set of elements designed to support the weight of the wagons. These elements include the superstructure, or line, which comprises rails, positioned on crosspieces and fasteners to fasten the rails to the crosspieces. The superstructure also comprises foundation base plates, which transmit the load supported by the rails to the ground and a ballast layer, optionally with a ballast sublayer. The ballast is a mat of crushed rocks, which ensures the uniform transmission to the platform of the stresses caused by the passage of railway vehicles. The ballast also makes it possible to stabilize the crosspieces and damp the mechanical and acoustic vibrations related to the passage of a train, which participates in the comfort of travelers onboard the trains. The ballast is further provided to drain rainwater so as to limit freezing problems on the tracks. The infrastructure is also distinguished, which is made up of a platform.
In practice, the elements of the track are designed to transmit the weight of the trains to the platform. The superstructure normally makes it possible to decrease the stress exerted on the platform by a factor of approximately 20,000 relative to that exerted at the points of contact between the wheels and the rail. This in particular makes it possible to prevent the platform from being deformed by the ballast.
The speed, the weight of the trains and the density of the traffic have increased considerably with the technological progress in recent years, which creates accelerated wear of the track components, in particular the ballast. The ballast must therefore be better maintained so that other components of the track are not damaged. Thus, the track repair operations, as well as the discontinuous and differential subsidence, are more frequent, which causes a considerable increase in maintenance costs and, in some cases, early renewal. Furthermore, it is considered that the maintenance of the ballast and the geometry of the track represent approximately half of the overall maintenance cost for railroad tracks, which is in particular due to the fact that the work is done during the night.
To withstand loads, components of the superstructure have evolved. Pre-stressed concrete crosspieces are now used, since they have a lifetime three times longer than that of wooden crosspieces. Heavier rail profiles and resilient fasteners have been developed to withstand the significant traffic. Sub-rail and sub-crosspiece base plates have been integrated into the line to damp vibrations. Geo-textiles and bituminous sublayers are applied to the base of the platform to guarantee the drainage and distribution of the loads.
However, there has been no progress regarding the protection of the ballast. To cope with the growing stresses, minimum thickness and hardness values have been imposed for the ballast. However, the latter remains the fastest-deteriorating element in a railroad track. This deterioration is accelerated by stresses due to traffic, but also by those due to maintenance work, such as packing, which have become more frequent.
The deterioration of the ballast is reflected in an attrition of the particles, i.e., a change in the grain size, as well as the rising of the ground, which is in particular caused by dynamic stresses created on the track by the passage of the trains. Two layers then appear, commonly referred to as “polluted ballast layer” and “intermediate” layer. The “polluted ballast” layer, which is located between the intermediate layer and the ballast layer, is formed by ballast as well as fines in particular coming from the attrition of the ballast. The “intermediate layer” is created by interpenetration of the layers of support ground with optional layers of material, such as broken stones, gravel, sand, or clinker, which result from the construction of the line or maintenance operations. This layer is fairly heterogeneous and contains, inter alia, more fines of the support ground and more particle fines than the ballast. The creation of these two layers within a track section poses mechanical and hydraulic stability problems, such as poor drainage, breaking of the grains of the “healthy” ballast layer, packing of the track, or deformation of the platform.
In order to cope with the deterioration of the tracks, a vast track renewal program has been launched in France. The problem that arises is to efficiently and cost-effectively establish a diagnosis of the railroad track condition, in particular aiming to detect the “polluted ballast” and “intermediate” layers. In light of the length of the tracks, the means for characterizing the ground must make it possible to monitor the large number of possible points along a track reliably, inexpensively and quickly. These means include the coring train, which makes it possible to withdraw samples of the track section. The thicknesses of the different layers of the ground are measured manually using a measuring stick or a meter stick and are transcribed manually on a worksite sheet. This monitoring technique is not the most appropriate, since only one coring train exists on the National Railway Network and the cost of this type of intervention is high. Indeed, four people, including a driver at a higher hourly rate, are generally necessary to perform the drilling. Furthermore, the coring is generally done between the two rails, whereas the most sensitive zone is below the crosspieces. This technique further lacks precision in measuring the thicknesses of the layers, since there is a risk of shifting during the raising of the core bit, especially for sandy materials. Lastly, this technique does not make it possible to measure the resistance of the ground.
A more elaborate technique consists of using a light dynamic penetrometer of the Panda type, which is a product marketed by the company SOL SOLUTION, and a geo-endoscopic test. The principle consists of measuring the strength of the ground as a function of the depth by pushing a train of rods of the penetrometer into the ground. The rods are next removed to make way for slotted tubes, in which an endoscope slides. A video is then recorded for the monitored point and is saved on a digital recorder. The interpretation of the thicknesses of the layers of ground is done manually using a meter stick, looking at the changes in nature of the soils on the screen of the geo-endoscope or the recorder. The results are next transcribed on a worksite sheet. The technician lastly performs a cross-analysis with the results obtained with the Panda penetrometer to make a diagnosis.
Although interesting, this monitoring technique is relatively time-consuming due to the manual transcription of the data, and there is no automatic and continuous measurement regarding the depth at which the images and video of the geo-endoscope are recorded. The diagnosis of the ground state is based in part on the technician's interpretation, which may lead to errors.
A similar monitoring technique is described in WO-A-2010/082002 and aims to characterize the different layers of a filtering medium, or filter, forming a water purification system. This filter comprises an upper part essentially formed by gravel, an intermediate filtration part formed by washed-out sand, and a lower part that is also gravel-based. The penetrometer test provides information on the compactness of the different layers of the filtering medium, while the endoscopic test makes it possible to verify the clogging and saturation states of the filtering medium. An automatic image analysis is done after the endoscopic test to characterize the different layers of the filtering medium, but no mention is made of an automatic measurement of the depth at which the images are taken. This measurement in fact appears to be useless in this case, since the geo-endoscopic test does not seek to determine the depth of the different layers of the filtering medium.